Zone serial not updated when deleting A record

I'm creating master DNS zones on ISPconfig3, and creating slaves on ISPConfig2. Both servers use bind9

Zones are replicated OK, but I'm having a little problem with zone updates.
When an A record is added, zonefile serial are increased, and slave server refresh its zonefile. But when an A record is deleted, record are deleted from zononefile, but zonefile serial are not changed, so slave server doesn't refresh